Optical modulator and optical transmission device using same

ABSTRACT

To provide a highly-reliable low-cost small optical modulator in which temperature drift is suppressed and an optical transmission device using the same. An optical modulator including an optical waveguide substrate 1 on which an optical waveguide is formed, a signal electrode which is provided on the optical waveguide substrate and applies an electric field to the optical waveguide, a termination substrate 3 provided with a termination resistor that terminates the signal electrode, and a housing 6 in which the optical waveguide substrate and the termination substrate are mounted, in which, in order to suppress conduction of heat generated from the termination resistor to the optical waveguide substrate through the housing, a groove 8 is formed in the housing 6 between the termination substrate 3 and the optical waveguide substrate 1.

TECHNICAL FIELD

The present invention relates to an optical modulator and an opticaltransmission device using the same and particularly to an opticalmodulator including a termination substrate and an optical transmissiondevice using the same.

BACKGROUND ART

Recently, in high-frequency/high-capacity optical fiber communicationsystems, an optical modulator for which an optical waveguide-formedsubstrate is used and an optical transmission device into which anoptical modulator as described above is combined are frequently used.Among them, optical modulators in which LiNbO₃ (referred to as “LN”)having an electro-optic effect is used in a substrate are more broadlyused in high-frequency/high-capacity optical fiber communication systemscompared with modulators of a semiconductor-based material such as InP,Si, or GaAs. In an optical modulator for which this LN is used, anoptical waveguide that guides light to the LN substrate in a confinedmanner is provided, and furthermore, electrodes that apply an electricfield to the optical waveguide are formed. In addition, in theelectrodes, an RF electrode portion that applies high-frequency signals,a DC electrode portion that applies low-frequency signals or DCvoltages, and the like are formed.

Regarding the modulation form of optical modulators forhigh-frequency/high-capacity optical fiber communication systems, inresponse to the recent trend of an increase in the transmissioncapacity, multilevel modulation or transmission formats achieved byincorporating polarization multiplexing into multilevel modulation suchas Quadrature Phase Shift Keying (QPSK) or Dual Polarization-QuadraturePhase Shift Keying (DP-QPSK) in which phase modulation is used hasbecome mainstream, replacing On-Off keying or the like of the relatedart. Furthermore, it has been also proposed to form multiple elementsusing a plurality of DP-QPSK chips and further increase the transmissioncapacity (for example, refer to Patent Literature No. 1).

As illustrated in FIG. 1, in a DP-QPSK optical modulator, an opticalwaveguide (not illustrated) in which two nest-type optical waveguidesincluding two Mach-Zehnder type optical waveguides are disposed isformed on a substrate (optical waveguide substrate) 1 of LN or the like.Furthermore, a plurality of signal electrodes 2 is provided on thesubstrate 1 in order to apply modulation signals to a modulation portionthat the respective Mach-Zehnder type optical waveguides constitute. Toeach of the signal electrodes 2, modulation signals are input through aconnector for electrical signal input 4. In addition, to a terminationof the signal electrode 2, a termination resistor 5 is connected. In acase in which the termination resistor 5 is provided to each of thesignal electrodes, there are cases in which a plurality of thetermination resistors 5 is provided on the same termination substrate 3as illustrated in FIG. 1, thereby reducing the size of the opticalmodulator. The substrate (optical waveguide substrate) 1 of LN or thelike and the termination substrate 3 are disposed in a housing 6 andthus form a package.

In order to operate the optical modulator at a high frequency, atravelling wave-type electrode constitution in which electrical signalsbeing input propagate through the signal electrodes is used. Signalfrequencies being input to the signal electrodes are high-frequencysignals in the microwave waveband, almost all of the input electricalenergy is consumed in the termination resistors 5 and converted to heatin the termination resistors.

In the DP-QPSK optical modulator, four modulation portions are provided.In order to deal with the phase modulation form with the presentconstitution, the optical modulator is driven at a voltage magnitudebeing twice (an electric power being four times) as large as that of theon-off keying form of a single modulator structure of the related art.Therefore, the electric power being consumed in the modulator becomes 16times or more as large as that consumed in a modulator having the singlemodulator structure of the related art. Furthermore, in order to meet arequirement of the size reduction of optical modulators, it is necessaryto dispose the termination substrate 3 close to the optical waveguidesubstrate 1, which creates a significant problem attributed to heatbeing generated from the termination substrate.

Furthermore, in the case of forming multiple elements which intends toincrease the transmission capacity by combining two or more DP-QPSKoptical modulator constitutions into the same housing as illustrated inFIG. 2, the amount of heat generated becomes 32 times or more as largeas that in the on-off keying form of the single modulator structure ofthe related art. Heat generated from the termination substratedeteriorates the temperature drift of the optical modulator. Inaddition, the generation of heat from the termination resistor causesthe deterioration of the termination resistors over time or theoccurrence of cracking, peeling, or the like and creates a seriousproblem of impairing the reliability of the optical modulator andoptical transmission devices using the same. Meanwhile, in FIG. 2,optical waveguide substrates 1 a and 1 b are disposed side by sidehorizontally, but there are cases in which a plurality of opticalwaveguide substrates is disposed to be laminated together as in PatentLiterature No. 1.

The influence of heat generated from the termination substrate has beenan underlying problem of almost all of the optical modulators having atravelling wave-type electrode constitution; however, in the relatedart, this problem has been rarely studied or dealt with. Instead, theinfluence of heat generated has been misunderstood as a temperaturechange of an environment in which the optical modulator is placed or theunstability of the optical modulator and has been handled as a problemof the deterioration of the intrinsic characteristics of the opticalmodulator such as temperature drift.

However, this influence has become particularly significant in (a) anoptical modulator in which the amplitude of input electrical signals islarge, (b) an optical modulator having a plurality of terminationresistors, and (c) an optical modulator in which termination resistorsare on the same substrate such as optical modulators having the DP-QPSKconstitution. Furthermore, this influence has become more serious as theoptical modulator has been reduced in size (d) and has been provided aplurality of elements (multiple elements) (e).

As a countermeasure for alleviating the above-described problem of heatgenerated from the termination resistors, it has been proposed toincrease the area of the termination resistors or provide aheat-conductive hole in the termination substrate as described in PatentLiterature No. 2. However, in these constitutions and methods, thetermination substrate becomes large, and the manufacturing costs alsoincrease, and thus the applicable usages are limited. Therefore, therehas been a desire for a solution which can be applied to a variety oftransmission formats and meets the requirement of size reduction or costreduction. In addition, there is a demand for a highly-reliable opticaltransmission device in which the temperature drift is suppressed bymounting an optical modulator to which a countermeasure to heatgenerated is provided.

CITATION LIST Patent Literature

[Patent Literature No. 1] Japanese Laid-open Patent Publication No.2015-69162

[Patent Literature No. 2] Japanese Laid-open Patent Publication No.2014-199302

SUMMARY OF INVENTION Technical Problem

An object of the present invention is, as described above, to provide anoptical modulator in which the influence of heat generated fromtermination resistors is suppressed and an optical transmission deviceusing the same. Particularly, the object is to suppress the generationof heat from the termination resistors which becomes more significant inoptical modulators having a plurality of signal inputs and a pluralityof termination resistors such as DP-QPSK optical modulators.Furthermore, the object is to provide an optical modulator whichsatisfies size reduction, the formation of multiple elements, and costreduction by providing an effective countermeasure to heat generated andan optical transmission device using the same.

Solution to Problem

In order to achieve the above-described object, an optical modulator ofthe present invention and an optical transmission device using the samehave technical characteristics as described below.

(1) An optical modulator including an optical waveguide substrate onwhich an optical waveguide is formed, a signal electrode which isprovided on the optical waveguide substrate and applies an electricfield to the optical waveguide, a termination substrate provided with atermination resistor that terminates the signal electrode, and a housingin which the optical waveguide substrate and the termination substrateare mounted, in which, in order to suppress conduction of heat generatedfrom the termination resistor to the optical waveguide substrate throughthe housing, a groove is formed in the housing between the terminationsubstrate and the optical waveguide substrate.

(2) An optical modulator including an optical waveguide substrate onwhich an optical waveguide is formed, a signal electrode which isprovided on the optical waveguide substrate and applies an electricfield to the optical waveguide, a termination substrate provided with atermination resistor that terminates the signal electrode, and a housingin which the optical waveguide substrate and the termination substrateare mounted, in which, in order to suppress conduction of heat generatedfrom the termination resistor to the optical waveguide substrate throughthe housing, a highly heat-conductive material is provided in a part ofthe housing in which the termination substrate is disposed.

(3) The optical modulator according to (2), in which, as the highlyheat-conductive material, at least one of complexes of Cu—W, Cu—Mo, andAl—SiC is used.

(4) An optical modulator including an optical waveguide substrate onwhich an optical waveguide is formed, a signal electrode which isprovided on the optical waveguide substrate and applies an electricfield to the optical waveguide, a termination substrate provided with atermination resistor that terminates the signal electrode, and a housingin which the optical waveguide substrate and the termination substrateare mounted, in which, in order to suppress conduction of heat generatedfrom the termination resistor to the optical waveguide substrate throughthe housing, the termination substrate is thinned.

(5) An optical modulator including an optical waveguide substrate onwhich an optical waveguide is formed, a signal electrode which isprovided on the optical waveguide substrate and applies an electricfield to the optical waveguide, a termination substrate provided with atermination resistor that terminates the signal electrode, and a housingin which the optical waveguide substrate and the termination substrateare mounted, in which, in order to suppress conduction of heat generatedfrom the termination resistor to the optical waveguide substrate throughthe housing, a level difference is provided between a mounting surfaceof the housing on which the termination substrate is mounted and amounting surface of the housing on which the optical waveguide substrateis mounted.

(6) The optical modulator according to (5), in which the terminationsubstrate is thinned, and the mounting surface of the housing on whichthe termination substrate is mounted is provided at a location higherthan the mounting surface of the housing on which the optical waveguidesubstrate is mounted.

(7) The optical modulator according to (5), in which the mountingsurface of the housing on which the termination substrate is mounted isprovided at a location lower than the mounting surface of the housing onwhich the optical waveguide substrate is mounted.

(8) An optical modulator including an optical waveguide substrate onwhich an optical waveguide is formed, a signal electrode which isprovided on the optical waveguide substrate and applies an electricfield to the optical waveguide, a termination substrate provided with atermination resistor that terminates the signal electrode, and a housingin which the optical waveguide substrate and the termination substrateare mounted, in which, in order to suppress conduction of heat generatedfrom the termination resistor to the optical waveguide substrate throughthe housing, a highly heat-conductive member is installed between thetermination substrate and the housing.

(9) The optical modulator according to (8), in which the terminationsubstrate is thinned.

(10) An optical modulator including an optical waveguide substrate onwhich an optical waveguide is formed, a signal electrode which isprovided on the optical waveguide substrate and applies an electricfield to the optical waveguide, a termination substrate provided with atermination resistor that terminates the signal electrode, and a housingin which the optical waveguide substrate and the termination substrateare mounted, in which, in order to suppress conduction of heat generatedfrom the termination resistor to the optical waveguide substrate throughthe housing, a level difference is provided between a mounting surfaceof the housing on which the termination substrate is mounted and amounting surface of the housing on which the optical waveguide substrateis mounted, and a part of the termination substrate is disposed so as toprotrude from a corner portion of the level difference.

(11) An optical modulator including an optical waveguide substrate onwhich an optical waveguide is formed, a signal electrode which isprovided on the optical waveguide substrate and applies an electricfield to the optical waveguide, a termination substrate provided with atermination resistor that terminates the signal electrode, and a housingin which the optical waveguide substrate and the termination substrateare mounted, in which, in order to suppress conduction of heat generatedfrom the termination resistor to the optical waveguide substrate throughthe housing, a level difference is provided between a mounting surfaceof the housing on which the termination substrate is mounted and amounting surface of the housing on which the optical waveguide substrateis mounted, and a groove is formed near the level difference in thehousing.

(12) The optical modulator according to (11), in which a part of thetermination substrate is disposed so as to protrude from a cornerportion of the level difference.

(13) An optical modulator including an optical waveguide substrate onwhich an optical waveguide is formed, a signal electrode which isprovided on the optical waveguide substrate and applies an electricfield to the optical waveguide, a termination substrate provided with atermination resistor that terminates the signal electrode, and a housingin which the optical waveguide substrate and the termination substrateare mounted, in which, in order to suppress conduction of heat generatedfrom the termination resistor to the optical waveguide substrate throughthe housing, a level difference is provided between a mounting surfaceof the housing on which the termination substrate is mounted and amounting surface of the housing on which the optical waveguide substrateis mounted, and, in an end portion of the termination substrate, the endportion closest to a corner portion of the level difference is disposedinward from the corner portion of the level difference.

(14) The optical modulator according to any one of (1) to (13), in whichat least a part of a bottom surface or a side surface of the terminationsubstrate is fixed to the housing using a heat-conductive adhesivecontaining an Ag filler.

(15) The optical modulator according to any one of (1) to (14), in whichat least two termination resistors are provided in the terminationsubstrate.

(16) The optical modulator according to any one of (1) to (15), in whichthe optical waveguide substrate includes a material selected fromLiNbO₃, InP, GaAs, and Si.

(17) An optical transmission device including the optical modulatoraccording to anyone of (1) to (16), a data generation portion thatgenerates data signals that are applied to the optical modulator, and alight source that inputs light waves to the optical modulator.

Advantageous Effects of Invention

The present invention enables the effective emission of heat beinggenerated from the termination resistor toward the housing side and,furthermore, the suppression of the heat emitted to the housing beingconducted toward the optical transmission device side that fixes thehousing and thus conducted to the optical waveguide substrate throughthe housing, and thus it becomes possible to reduce the influence of theheat on the optical waveguide substrate. Therefore, it is possible toprovide a highly-reliable low-cost small optical modulator in whichtemperature drift is suppressed. In addition, it also becomes possibleto provide an optical transmission device in which temperature drift ishighly reliably suppressed by mounting the optical modulator of thepresent invention in the optical transmission device.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a plan view illustrating an example of a DP-QPSK opticalmodulator.

FIG. 2 is a plan view illustrating an example in which two DP-QPSKoptical modulators are mounted.

FIG. 3 is a cross-sectional view in an X-X′ direction in FIG. 1 or FIG.2 and a view illustrating an example of an optical modulator of therelated art.

FIG. 4 is a cross-sectional view illustrating a first example accordingto an optical modulator of the present invention.

FIG. 5 is a cross-sectional view illustrating a second example accordingto the optical modulator of the present invention.

FIG. 6 is a cross-sectional view illustrating a third example accordingto the optical modulator of the present invention.

FIG. 7 is a cross-sectional view illustrating a fourth example accordingto the optical modulator of the present invention.

FIG. 8 is a cross-sectional view illustrating a fifth example accordingto the optical modulator of the present invention.

FIG. 9 is a cross-sectional view illustrating a sixth example accordingto the optical modulator of the present invention.

FIG. 10 is a cross-sectional view illustrating a seventh exampleaccording to the optical modulator of the present invention.

FIG. 11 is a cross-sectional view illustrating an eighth exampleaccording to the optical modulator of the present invention.

FIG. 12 is a cross-sectional view illustrating a ninth example accordingto the optical modulator of the present invention.

FIG. 13 is a cross-sectional view illustrating a tenth example accordingto the optical modulator of the present invention.

FIG. 14 is a view illustrating an example in which the optical modulatorof the present invention is combined into an optical transmissiondevice.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an optical modulator according to an aspect of the presentinvention will be described in detail using FIGS. 3 to 14.

FIG. 3 is a cross-sectional view in an X-X′ direction in FIG. 1 or FIG.2 and a cross-sectional view of a constitution example of the relatedart illustrating a state of an optical waveguide substrate 1 and atermination resistor substrate 3 mounted. As the optical waveguidesubstrate 1, there is an optical waveguide substrate for which adielectric body such as LiNbO₃ or LiTaO₃ or a semiconductor such as InPor Si is used. In the optical modulator of the present invention, thematerial of the optical waveguide substrate is not particularly limited,and a substrate of a well-known dielectric body or semiconductor of therelated art can be used. In addition, in the formation of an opticalwaveguide, it is possible to use a well-known technique such as a methodof forming an optical waveguide by thermally diffusing Ti into adielectric body substrate of LiNbO₃, a ridge-type optical waveguide, orthe like.

As the constitution of the optical waveguide, it is possible to use avariety of shapes of an optical waveguide such as a single Mach-Zehndertype optical waveguide, a so-called nest-type optical waveguide in whichtwo Mach-Zehnder type optical waveguides are disposed in a nest form,and furthermore, an optical waveguide in which two nest-type opticalwaveguides including two Mach-Zehnder type optical waveguides aredisposed like the DP-QPSK optical modulator also disclosed by PatentLiterature No. 1.

In addition, regarding the number of signal electrodes that apply inputelectrical signals to a modulation portion of the optical waveguide, thepresent invention is not limited to a DP-QPSK optical modulatorincluding four signal electrodes as illustrated in FIG. 1, and can beapplied to a variety of optical modulators such as a single-type opticalmodulator (one signal electrode for modulation), a dual-type opticalmodulator (two signal electrodes for modulation), and a DQPSKconstitution modulator (two signal electrodes for modulation).Particularly, as the number of signal electrodes increases, the presentinvention can be more effectively applied and, for example, the presentinvention can be particularly effectively applied to a multiple elementconstitution in which two DP-QPSK optical modulators are combinedtogether and eight input electrical signals are provided as illustratedin FIG. 2.

As illustrated in an example of the related art of FIG. 3 as well, thetermination substrate 3 may be disposed in a state in which thetermination substrate is as close to the optical waveguide substrate 1as possible so as to prevent the deterioration of the reflection andtransmission characteristics of high-frequency signals being input sothat the heights of the top surfaces substantially match each other withno level differences. Therefore, in the related art, for a surface of ahousing 6 on which the termination substrate 3 and the optical waveguidesubstrate 1 are mounted, a flat constitution as illustrated in FIG. 3has been employed unless there is a particular request from theviewpoint of the ease of mounting the substrates and elements or theease of working the housing. In addition, generally, a wire of thetermination substrate 3 and a wire of the optical waveguide substrateare connected by a Au bonding or the like as indicated by a referencesign 7 so that input electrical signals propagate to the terminationsubstrate 3 as efficiently as possible.

The element number of termination resistors that are formed on thetermination substrate 3 is appropriately selected depending on theconstitution of the optical modulator, the electrode design, and thehousing design. In the optical modulator having the DP-QPSK constitutionof FIG. 1, an example of four termination resistors formed on the sametermination substrate is illustrated, but it is also possible toconstitute the optical modulator using a plurality of terminationsubstrates. Furthermore, it is also possible to constitute the fourtermination resistors in FIG. 1 as electric circuits for terminationincluding one or two termination resistors and form these electriccircuits on the same substrate. Recently, in order to reduce the size ofthe optical modulator, there have been many cases in which a pluralityof termination resistors is formed on the same substrate. The presentinvention can be effectively applied to an optical modulator in whichtwo or more termination resistors are used as illustrated in FIG. 1 orFIG. 2, particularly, a case in which a plurality of terminationresistors is disposed on the same termination substrate.

FIG. 4 is a cross-sectional view illustrating a first example of theoptical modulator of the present invention. Similar to FIG. 3, FIG. 4corresponds to a cross-sectional view in the X-X′ direction in FIG. 1 orFIG. 2.

In the present example, a groove 8 is formed in the housing 6 betweenthe termination substrate 3 and the optical waveguide substrate 1,thereby forming a constitution in which the conduction of heat generatedfrom the termination substrate through the housing and the consequentinfluence on the optical waveguide substrate are suppressed.

Generally, the housing is selected from metallic materials such as SUSor Kovar in consideration of cutting workability or an assembly stepsuch as laser welding or sealing. These metallic materials have a morehighly heat-conductive property than the termination substrate and thelike, and heat generated from the termination resistors is conducted tothe optical waveguide substrate through the metal housing 6. In order toreduce and suppress this influence, the groove 8 is formed in a part ofthe housing.

As the width of the groove (the width of the groove 8 in FIG. 4 in thehorizontal direction) increases, the influence of termination heatgeneration can be further suppressed; however, in order to increase thehousing stiffness in spite of a small size and prevent the groove frominfluencing the mounting of the optical waveguide substrate and thetermination substrate, the width may be set to approximately 0.1 mm to 1mm. In addition, as the depth of the groove increases, the influence ofheat generated from the termination resistors can be further suppressed;however, due to the limitation on the housing thickness, the depth maybe set in a range of approximately 0.5 mm to 2 mm.

FIG. 5 is a view illustrating a second example of the optical modulatorof the present invention.

In the present example, a highly heat-conductive material 9 is providedin the surface of the housing 6 on which the termination substrate 3 ismounted. The highly heat-conductive material refers to a material havinga higher heat-conductive property than the housing material, and, morespecifically, Cu—W, Cu—Mo, and Al—SiC complexes having a higherheat-conductive property than SUS or Kovar are selected.

This highly heat-conductive material 9 is installed below thetermination substrate 3 so as to more effectively and more selectivelyemit heat generated from the termination substrate to the outside of thehousing and may be formed so as to reach the outside of the housing. Theabove-described constitution is formed by, first, forming a penetrationportion in a part of the housing and then fixing a highlyheat-conductive material having substantially the same shape as that ofthe penetration portion using silver brazing, gold tin, or the like.Meanwhile, in order to enhance the air tightness in the housing, it isalso possible to form a constitution in which a recess portion isprovided in an inside bottom surface or the like without completelypenetrating the housing and the highly heat-conductive material isdisposed in the recess portion.

FIG. 6 is a view illustrating a third example of the optical modulatorof the present invention.

In the present invention, the thickness of the termination substrate 3is decreased (thinned) in the example. As the material that is used forthe termination substrate, ceramic materials such as alumina havingexcellent high-frequency characteristics are frequently used since thetermination substrate is used at high frequencies such as the microwavewaveband. Generally, these ceramic materials have a poorerheat-conductive property than metallic materials, and thus it iseffective to thin the termination substrate in order to efficientlytransfer heat generated from the termination substrate to the outside ofthe substrate.

The thickness of the thinned termination substrate 3 needs to beselected in comprehensive consideration of the strength, heat-conductiveproperty, size, and the like of the material used for the terminationsubstrate and may be thinner than the thickness (generally, 0.5 mm to2.0 mm) of a chip in which at least an optical waveguide is formed andwhich is installed beside the termination substrate. When the emissionof heat generated from the termination resistors is taken into account,it is advantageous to decrease the thickness; however, when themechanical strength or the impedance design of the termination resistorsis taken into account, the thickness is preferably set in a range from0.05 mm to 0.8 mm.

Heat generated from the termination resistors is efficiently conductedtoward the housing since the termination substrate is formed thin.Therefore, the termination resistors becoming hot is suppressed, and itis possible to solve reliability problems such as the deterioration ofthe termination resistors over time or cracking, peeling, or the like.In addition, the thickness of the housing on which the terminationsubstrate is mounted is set to be thicker than that of a portion onwhich the optical waveguide substrate is mounted as illustrated in FIG.6, and thus it becomes easy to diffuse heat toward the thick portion ofthe housing, and it is also possible to effectively reduce thetemperature drift of the optical modulator.

A level difference 60 provided in the mounting surface of thetermination substrate and the optical waveguide substrate as in thepresent example creates an asymmetry of heat conduction, and this leveldifference constitution alone has an action of reducing the influence ofthe conduction of heat generated from the termination resistors to theoptical waveguide substrate.

FIG. 7 is a view illustrating a fourth example of the optical modulatorof the present invention.

In the present invention, the thickness of a bottom surface (refer to areference sign 61) of the housing 6 on which the termination substrate 3is mounted is set to be thin in the example. In this example, thethickness of a bottom surface 61 of the housing 6 located below thetermination substrate generating heat is thinned in order to efficientlyemit heat generated from the termination substrate to the outside of thehousing, thereby forming a constitution in which heat is efficientlyemitted to a structure of an optical transmission device that fixes thehousing 6.

As the thickness of this portion decreases, heat can be more efficientlyemitted to the outside; however, due to the limitations of the housingstiffness maintained at a certain level and the thickness of thehousing, and the like, the thickness may be set in a range ofapproximately 0.2 mm to 2 mm. In this case as well, a level differenceis formed in the mounting surface of the termination substrate 3 and theoptical waveguide substrate 1. Since the heights of the top surfaces ofthe two substrates are matched each other, and thus the thickness of thetermination substrate may be set to be thick as in FIG. 7 or thethickness of the optical waveguide substrate may be set to be thin whilekeeping the thickness of the termination substrate unchanged.

FIG. 8 is a view illustrating a fifth example of the optical modulatorof the present invention.

In the present example, in addition to the constitution of FIG. 7, ahighly heat-conductive material 10 is provided below the terminationsubstrate 3. In the present constitution, a constitution in which theconduction of heat generated from the termination substrate to thehousing from the termination substrate is efficiently accelerated usinga highly heat-conductive body is employed. In the example of FIG. 8, aconstitution example in which the thickness of the termination substrateremains unchanged and a highly heat-conductive body as thick as thethinned thickness of the housing is provided is illustrated, but aconstitution in which the termination substrate is further thinned andthe highly heat-conductive body is provided becomes a more preferredexample. The material of the highly heat-conductive body, the thinnedthickness, and the like are as described above.

FIG. 9 is a view illustrating a sixth example of the optical modulatorof the present invention.

A constitution of the present example is a constitution in which nolevel difference is provided in the housing as in the example of therelated art in contrast to the constitution illustrated in FIG. 8 inwhich the level difference is provided in the mounting surface of thetermination substrate and the optical waveguide substrate. In thisconstitution, the termination substrate 3 is thinned, and the highlyheat-conductive material 10 is installed, whereby it is possible torealize both the suppression of the influence of heat generation to acertain extent and the provision of both the ease of mounting thetermination substrate and the optical waveguide substrate and the easeof manufacturing the housing.

FIG. 10 is a view illustrating a seventh example of the opticalmodulator of the present invention.

In the example of the present invention, the termination substrate 3 isthinned, and a part 30 of the termination substrate 3 protrudes from amounting surface 63. In this constitution, heat is efficiently conductedto the metal housing 6 from the termination substrate 3 generating heatand, due to the constitution in which the part of the terminationsubstrate protrudes, a space in proportion to the level difference inthe housing and the protrusion degree of the termination substrate 3 isformed between the termination substrate and the optical waveguidesubstrate, and thus the influence of thermal conduction through thehousing is further suppressed.

As the protrusion degree increases, the effect of suppressing the heatinfluence becomes stronger, realistically, it is necessary to considerthe mounting property of the termination substrate or the influence onAu bonding, but the protrusion degree has a wide range of choice and isset in a range of approximately 10 μm to 2 mm. Even in the constitutionin which the part of the termination substrate 3 protrudes as describedabove, the top surface of the termination substrate 3 and the topsurface of the optical waveguide substrate 1 have substantially the sameheight, and furthermore, in a case in which the gap between thetermination substrate and the optical waveguide substrate is narrow, itis possible to avoid the high-frequency characteristics beingsacrificed. The gap between both substrates in this case may be set to100 μm or less.

FIG. 11 is a view illustrating an eighth example of the opticalmodulator of the present invention.

In the constitution of the present invention, in addition to theconstitution of FIG. 10, furthermore, the groove 8 is formed in thehousing, and the influence of heat generated from the terminationresistor on the optical waveguide substrate is suppressed. The thicknessand the protrusion degree of the termination substrate, the width ordepth of the groove, and the like are the same as the above-describedcontents and thus will not be described again herein. This constitutionof FIG. 11 has the strongest effect of suppressing the heat influenceamong the several examples listed.

FIG. 12 is a view illustrating a ninth example of the optical modulatorof the present invention.

In the present example, an end portion 31 of the thinned terminationsubstrate 3 is disposed inward from a corner portion 64 of a housingmounting surface provided with a level difference. In this constitution,a large amount of heat generated from the termination substrate 3 isconducted to a housing section located below the termination substrateand only a small amount of the heat is conducted to a portion on whichthe optical waveguide substrate is mounted. Furthermore, the gap betweenthe termination substrate 3 and the optical waveguide substrate 1becomes broad, and thus a constitution in which the heat influence canbe further reduced is formed.

In addition, the present constitution has an effect of enlarging themounting tolerance of the termination substrate 3 and also reducing thenumber of assembly man-hours or suppressing an adverse effect (cracking,chipping, or the like) by the interference with the optical waveguidesubstrate 1. As the distance from the corner portion 64 of the leveldifference in the housing to the end portion 31 of the terminationsubstrate increases, the effect of reducing the heat influence becomesstronger; however, when adverse effects on the reflection ortransmission characteristics of high-frequency signals are taken intoaccount, the distance may be set in a range of approximately 100 μm.

FIG. 13 is a view illustrating a tenth example of the optical modulatorof the present invention.

In the present example, a bottom surface or a side surface of thetermination substrate 3 generating heat is fixed using a heat-conductiveadhesive 11 containing an Ag filler. Compared with ordinary adhesives,the heat-conductive adhesive containing an Ag filler has a viscositythat is more easily adjusted and is more easily cured, enables thetermination substrate 3 to be mounted on the housing 6 with a higheraccuracy and a more favorable workability, and is also capable ofstabilizing the high-frequency characteristics due to its high electricconductive property. In addition, when the heat-conductive adhesivecontaining an Ag filler is used in both the bottom surface and the sidesurface of the termination substrate, it becomes possible to moreefficiently conduct heat generated from the termination substrate.Furthermore, the adhesive containing an Ag filler also have a variety ofadvantages so that the heat-conductive adhesive is easily formed thickand high on the side surface of the termination substrate in orderparticularly to efficiently conduct heat in the housing from the sidesurface of the termination substrate.

When the constitutions of the present invention as described above areemployed, the mounting workability of the termination substratesignificantly improves, and, at the same time, it is possible to providean optical modulator which has excellent high-frequency characteristicsand in which termination heat generation is effectively reduced.

FIG. 14 is a constitution example of an optical transmission device inwhich the optical modulator of the present invention is mounted.

A basic constitution of the optical transmission device includes a lightsource that generates light waves that are introduced into the opticalmodulator, the optical modulator, a data generation portion that appliessignals to the optical modulator, and an optical fiber for guidingmodulated light generated from the optical modulator to the outside.

When the optical transmission device begins to be driven, temperaturedrift occurs in the optical modulator. In order to stabilize thetransmission characteristics with a high quality, it becomes necessaryto operate the optical modulator while controlling the optical modulatorso as to hold the operation point of the optical modulator in anappropriate state. In the related art, this temperature drift has beenconsidered as the influence of heat generation in peripheral devices ofthe optical modulator such as the light source or the data generationportion.

However, in DP-QPSK optical modulators, small optical modulators, andthe like, there are cases in which large temperature drift occursimmediately after the beginning of the driving of an opticaltransmission device and the transmission characteristics of the opticaltransmission device become significantly unstable. As a result of thepresent inventors' intensive studies regarding the causes, it has beenclarified that the causes cannot be explained with heat generation inperipheral devices or changes in ambient temperatures which have beenconsidered as the causes in the related art. In addition, the presentinventors found that the generation of heat from termination resistorsin the optical modulator has an influence and found that the temperaturedrift is a phenomenon that occurs particularly significantly,particularly, in an optical modulator constitution to which a pluralityof high-frequency signals is input, in a case in which a plurality oftermination resistors is formed on the same substrate, a case in whichthe amplitude of input signals is large, and furthermore, a case inwhich optical modulators are small.

Regarding this problem, when the optical modulator provided by thepresent invention is disposed in the optical transmission device, it ispossible to reduce temperature drift caused by the generation of heatfrom the termination resistors and stabilize the transmissioncharacteristics with a high quality.

The above-described examples are not limited to the constitution ofDP-QPSK optical modulators in which a LiNbO₃ substrate is used, and thepresent invention can be applied regardless of the modulation form aslong as optical modulators have termination resistors, and thegeneration of heat from the termination resistors have an influence onthe characteristics of the optical modulators. In addition, the opticalwaveguide substrate may be an optical waveguide substrate of asemiconductor-based material such as InP or Si, and it is needless tosay that, even in a case in which a LiNbO₃ substrate is used, thepresent invention can be applied regardless of crystal orientations suchas Xcut or Zcut.

In addition, in the above-described examples, examples of thetermination substrate in which only the termination resistors are formedon the substrate have been described, but capacitors, other electroniccomponents, penetrating conductors, or multilayered electronic circuitsmay be formed on the termination substrate. In addition, it is needlessto say that the termination resistors may be formed a differenttermination substrate.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, it is possibleto provide a highly-reliable low-cost small optical modulator in whichtemperature drift is suppressed. In addition, it is possible to providean optical transmission device in which the optical modulator of thepresent invention is mounted and temperature drift is highly reliablysuppressed.

REFERENCE SIGNS LIST

-   -   1: OPTICAL WAVEGUIDE SUBSTRATE    -   2: SIGNAL ELECTRODE    -   3: TERMINATION SUBSTRATE    -   4: CONNECTOR FOR ELECTRICAL SIGNAL INPUT    -   5: TERMINATION RESISTOR (HEAT GENERATION PORTION)    -   6: HOUSING    -   7: ELECTRIC CONNECTION WIRE (Au BONDING)    -   8: HOUSING SECTION GROOVE    -   9: HOUSING LEVEL DIFFERENCE    -   10: HIGHLY HEAT-CONDUCTIVE MATERIAL    -   11: HEAT-CONDUCTIVE ADHESIVE

1. An optical modulator comprising: an optical waveguide substrate onwhich an optical waveguide is formed; a signal electrode which isprovided on the optical waveguide substrate and applies an electricfield to the optical waveguide; a termination substrate provided with atermination resistor that terminates the signal electrode; and a housingin which the optical waveguide substrate and the termination substrateare mounted, wherein, in order to suppress conduction of heat generatedfrom the termination resistor to the optical waveguide substrate throughthe housing, a groove is formed in the housing between the terminationsubstrate and the optical waveguide substrate.
 2. An optical modulatorcomprising: an optical waveguide substrate on which an optical waveguideis formed; a signal electrode which is provided on the optical waveguidesubstrate and applies an electric field to the optical waveguide; atermination substrate provided with a termination resistor thatterminates the signal electrode; and a housing in which the opticalwaveguide substrate and the termination substrate are mounted, wherein,in order to suppress conduction of heat generated from the terminationresistor to the optical waveguide substrate through the housing, ahighly heat-conductive material is provided in a part of the housing inwhich the termination substrate is disposed.
 3. The optical modulatoraccording to claim 2, wherein, as the highly heat-conductive material,at least one of complexes of Cu—W, Cu—Mo, and Al—SiC is used.
 4. Anoptical modulator comprising: an optical waveguide substrate on which anoptical waveguide is formed; a signal electrode which is provided on theoptical waveguide substrate and applies an electric field to the opticalwaveguide; a termination substrate provided with a termination resistorthat terminates the signal electrode; and a housing in which the opticalwaveguide substrate and the termination substrate are mounted, wherein,in order to suppress conduction of heat generated from the terminationresistor to the optical waveguide substrate through the housing, thetermination substrate is thinned.
 5. An optical modulator comprising: anoptical waveguide substrate on which an optical waveguide is formed; asignal electrode which is provided on the optical waveguide substrateand applies an electric field to the optical waveguide; a terminationsubstrate provided with a termination resistor that terminates thesignal electrode; and a housing in which the optical waveguide substrateand the termination substrate are mounted, wherein, in order to suppressconduction of heat generated from the termination resistor to theoptical waveguide substrate through the housing, a level difference isprovided between a mounting surface of the housing on which thetermination substrate is mounted and a mounting surface of the housingon which the optical waveguide substrate is mounted.
 6. The opticalmodulator according to claim 5, wherein the termination substrate isthinned, and the mounting surface of the housing on which thetermination substrate is mounted is provided at a location higher thanthe mounting surface of the housing on which the optical waveguidesubstrate is mounted.
 7. The optical modulator according to claim 5,wherein the mounting surface of the housing on which the terminationsubstrate is mounted is provided at a location lower than the mountingsurface of the housing on which the optical waveguide substrate ismounted.
 8. An optical modulator comprising: an optical waveguidesubstrate on which an optical waveguide is formed; a signal electrodewhich is provided on the optical waveguide substrate and applies anelectric field to the optical waveguide; a termination substrateprovided with a termination resistor that terminates the signalelectrode; and a housing in which the optical waveguide substrate andthe termination substrate are mounted, wherein, in order to suppressconduction of heat generated from the termination resistor to theoptical waveguide substrate through the housing, a highlyheat-conductive member is installed between the termination substrateand the housing.
 9. The optical modulator according to claim 8, whereinthe termination substrate is thinned.
 10. An optical modulatorcomprising: an optical waveguide substrate on which an optical waveguideis formed; a signal electrode which is provided on the optical waveguidesubstrate and applies an electric field to the optical waveguide; atermination substrate provided with a termination resistor thatterminates the signal electrode; and a housing in which the opticalwaveguide substrate and the termination substrate are mounted, wherein,in order to suppress conduction of heat generated from the terminationresistor to the optical waveguide substrate through the housing, a leveldifference is provided between a mounting surface of the housing onwhich the termination substrate is mounted and a mounting surface of thehousing on which the optical waveguide substrate is mounted, and a partof the termination substrate is disposed so as to protrude from a cornerportion of the level difference.
 11. An optical modulator comprising: anoptical waveguide substrate on which an optical waveguide is formed; asignal electrode which is provided on the optical waveguide substrateand applies an electric field to the optical waveguide; a terminationsubstrate provided with a termination resistor that terminates thesignal electrode; and a housing in which the optical waveguide substrateand the termination substrate are mounted, wherein, in order to suppressconduction of heat generated from the termination resistor to theoptical waveguide substrate through the housing, a level difference isprovided between a mounting surface of the housing on which thetermination substrate is mounted and a mounting surface of the housingon which the optical waveguide substrate is mounted, and a groove isformed near the level difference in the housing.
 12. The opticalmodulator according to claim 11, wherein a part of the terminationsubstrate is disposed so as to protrude from a corner portion of thelevel difference.
 13. An optical modulator comprising: an opticalwaveguide substrate on which an optical waveguide is formed; a signalelectrode which is provided on the optical waveguide substrate andapplies an electric field to the optical waveguide; a terminationsubstrate provided with a termination resistor that terminates thesignal electrode; and a housing in which the optical waveguide substrateand the termination substrate are mounted, wherein, in order to suppressconduction of heat generated from the termination resistor to theoptical waveguide substrate through the housing, a level difference isprovided between a mounting surface of the housing on which thetermination substrate is mounted and a mounting surface of the housingon which the optical waveguide substrate is mounted, and, in an endportion of the termination substrate, the end portion closest to acorner portion of the level difference is disposed inward from thecorner portion of the level difference.
 14. The optical modulatoraccording to any one of claims 1 to 13, wherein at least a part of abottom surface or a side surface of the termination substrate is fixedto the housing using a heat-conductive adhesive containing an Ag filler.15. The optical modulator according to any one of claims 1 to 4413,wherein at least two termination resistors are provided in thetermination substrate.
 16. The optical modulator according to any one ofclaims 1 to 4413, wherein the optical waveguide substrate comprises amaterial selected from LiNbO₃, InP, GaAs, and Si.
 17. An opticaltransmission device comprising: the optical modulator according to anyone of claims 1 to 13; a data generation portion that generates datasignals that are applied to the optical modulator; and a light sourcethat inputs light waves to the optical modulator.